The kidneys fulfil many functions, including the removal of water, the excretion of catabolites (or waste from the metabolism, for example urea and creatinine), the regulation of the concentration of the electrolytes in the blood (e.g. sodium, potassium, magnesium, calcium, bicarbonates, phosphates, chlorides) and the regulation of the acid/base equilibrium within the body, which is obtained in particular by the removal of weak acids and by the production of ammonium salts.
In individuals who have lost the use of their kidneys, since these excretion and regulation mechanisms no longer work, the body accumulates water and waste from the metabolism and exhibits an excess of electrolytes, as well as, in general, acidosis, the pH of the blood plasma shifting downwards, below 7.35 (the blood pH normally varies within narrow limits of between 7.35 and 7.45).
In order to overcome renal dysfunction, resort is conventionally made to a blood treatment involving extracorporeal circulation through an exchanger having a semipermeable membrane (dialyzer) in which the patient's blood is circulated on one side of the membrane and a dialysis liquid, comprising the main electrolytes of the blood in concentrations close to those in the blood of a healthy subject, is circulated on the other side.
Furthermore, a pressure difference is created between the two compartments of the dialyzer which are delimited by the semipermeable membrane, so that a fraction of the plasma fluid passes by ultrafiltration through the membrane into the compartment containing the dialysis liquid.
The blood treatment which takes place in a dialyzer as regards waste from the metabolism and electrolytes results from two mechanisms of molecular transport through the membrane.
On the one hand, the molecules migrate from the liquid where their concentration is higher to the liquid where their concentration is lower. This is diffusive transport.
On the other hand, certain catabolites and certain electrolytes are entrained by the plasma fluid which filters through the membrane under the effect of the pressure difference created between the two compartments of the exchanger. This is convective transport.
Three of the abovementioned functions of the kidney, namely the removal of water, the excretion of catabolites and the regulation of the electrolytic concentration of the blood, are therefore performed in a conventional blood treatment device by the combination of dialysis and blood filtration (this combination is referred to as hemodialysis).
As regards the regulation of the acid/base equilibrium inside the body, the approach adopted to overcome renal deficiency is to act on a mechanism by which the acid/base equilibrium inside the body is regulated, this mechanism consisting of the buffer systems of the blood, the main one of which comprises carbonic acid, as a weak acid, associated with its alkali salt, bicarbonate. This is why, in order to correct acidosis in a patient suffering from renal insufficiency, he is administered with bicarbonate via the vascular route, directly or indirectly, during a hemodialysis session.
Moreover, it must be underlined that sodium is the main ionic solute of extracellular volume. From literature search and according to the main opinion leaders in the dialysis field, the determination of dialysis fluid sodium concentration to be used during the dialysis treatment appears as one of the major challenges of dialysis prescription.
The dialysis fluid sodium concentration significantly affects the sodium balance and the intracellular hydration of the patient with implications on hemodialysis tolerance and also long term patient survival.
Hypertonic dialysis fluid sodium prescription will result in a positive sodium balance followed by a water shift from the intracellular to extracellular compartment. The intracellular dehydration increases vasopressin release and provokes thirst with the consequence of a greater inter-dialytic weight gain and hypertension.
On the contrary, a dialysis fluid sodium concentration that is too low (i.e., hypotonic) will provoke a negative sodium gradient with a water shift in the intracellular compartment, which is responsible for intra-dialytic cramps, headache, hypovolemia and risk of hypotension.
One of current opinions is the idea that sodium balance should be maintained null during a dialysis treatment: this is based on the so-called “sodium set point” theory, according to which both healthy subjects and dialysis patients tend to maintain a stable extra-cellular sodium concentration.
As above mentioned, sodium is removed during dialysis through convection and diffusion. The main sodium removal process during dialysis is convective. If we assume that the ultrafiltrate fluid is basically isotonic, convection does not change the tonicity of the extracellular fluid.
There is a need to help the physician to prescribe a “physiological” dialysis fluid composition to treat the patient.
Moreover, a second need is to have a bio-sensing-based therapy which is easy to use and designed also for operators not very skilled or working in crowded and very busy dialysis rooms.
To at least partly solve the above mentioned drawbacks, document U.S. Pat. No. 4,508,622 teaches a dialysis device in which the electrolyte composition of the untreated and treated fluids routed through the dialyzer may be determined and the composition of the dialysis solution adapted to the patient's requirements.
A first electrolyte detector (conductivity cell) is provided upstream of the dialyzer and a second electrolyte detector (conductivity cell) is provided downstream of the dialyzer. Each detector is coupled to a readout element through which both of the values of the dialysis solution may be observed and eventually controlled. In more detail, the apparatus according to U.S. Pat. No. 4,508,622 consists essentially of a unit for production of the dialysis solution and a dialyzer connected to the unit and followed downstream by a pump to produce a vacuum in the dialyzer on the side of the dialysis fluid. The detector mounted upstream of the dialyzer, and connected with a control unit, measures the conductivity of the total dialysis solution.
A second detector is mounted downstream of dialyzer and is connected with a comparator which is, in turn, connected to a differentiation unit. A control signal is provided by the differentiation unit to control unit if there is a difference in the differentiation unit that deviates from the predetermined nominal value.
During dialysis fluid circulation, if detector generates a signal to the evaluation unit and subsequently to the differentiation unit which deviates by a certain amount from the signal generated by detector, i.e., a difference in value appears which deviates from the predetermined value for differentiation unit, the difference unit activates the control unit, which in turn switches concentrate pump on or off as a function of the higher or lower concentration in the dialysis solution to be produced. A treatment in which the dialysis fluid has the same conductivity of the blood and of the spent dialysis fluid, is one of the described implementations.
However, the dialysis fluid and the blood reach the same conductivity after a certain time lapse which clearly affects the pre-dialytic plasma sodium content. Therefore, the method described in U.S. Pat. No. 4,508,622 in not properly an ‘isoconductive’ dialysis treatment.
In any case, ‘isoconductive’ dialysis has been shown to lead to undesired sodium loading in the patient.
Moreover, the prior art devices include dialysis apparatus wherein the conductivity of dialysis fluid is controlled in order to reach a desired post-dialysis plasmatic conductivity, i.e. conductivity (or sodium concentration) of the patient's blood at the end of the dialysis treatment.
It is known, for example from EP 1389475, a dialysis apparatus provided with a conductivity system that computes the dialysis fluid conductivity (corresponding to the dialysis fluid sodium concentration) from periodic measurements of the sodium blood concentration allowing the sodium level of the patient to reach a prescribed end-of-session value.
This dialysis apparatus includes a bag and a pump for infusing a patient with an infusion solution containing sodium at a determined and known concentration.
A structure for determining the sodium concentration [Na+]dial of the dialysis liquid is also provided so that the patient's body tends towards a desired sodium concentration [Na+]des, as a function of the dialysance D for sodium of the dialyser, of the desired sodium concentration [Na+]des inside the patient's body, of the infusion flow rate and of the sodium concentration [Na+]sol of the infusion solution.
A control unit drives the pump for regulating the sodium concentration of the dialysis liquid such that this concentration is equal (tends towards) to the determined concentration [Na+]dial.
As previously mentioned, one of the problems of the dialysis apparatus of the discussed prior art is presently the choice of the appropriate post-dialysis plasmatic conductivity target.
EP 2377563 discloses a dialysis apparatus comprising a blood treatment unit with an online preparation device for preparing a dialysis fluid containing sodium and comprising a dialysis preparation section for regulating the concentration of sodium in the dialysis fluid. The blood circuit is configured to circulate extracorporeal blood through the blood chamber; control means determines a value representative of the sodium concentration in the blood and are programmed for driving the dialysis preparation section as a function of the determined plasma sodium value, such that the substance concentration in the dialysis fluid tends towards the substance concentration in the blood.
The plasma sodium content is determined by measuring the inlet and outlet conductivities of the dialysis fluid upstream and downstream the dialyzer, by then changing the conductivity upstream the filter by a prefixed step and measuring a second time the inlet and outlet conductivities of the dialysis fluid upstream and downstream the dialyzer with the modified conductivity value.
With the methods described, for example in EP 547025 or in EP 920877, it is possible to determine the plasma conductivity and thereby to properly regulate the dialysis fluid preparation section.
The described system however changes the blood conductivity and tonicity since the dialysis fluid enters into contact and exchange significantly with blood before a plasma conductivity calculation; the effect on plasma conductivity is in an amount proportional to the difference between blood and dialysis fluid conductivities.
Finally, document U.S. Pat. No. 8,182,692 describes a dialysis apparatus providing a treatment in which a dialysis fluid having a sodium concentration substantially equal to the estimated current sodium concentration in the patient's blood is performed by placing the dialysis fluid in communication with the patient's blood across the semi-permeable membrane to perform a dialysis treatment on the patient's blood without substantially altering the sodium concentration of the patient's blood during the performance of the dialysis treatment.
In more detail, a solution supply device, containing a conductivity-testing solution, is selectively placed in communication with dialyzer and the blood flowing therein.
According to this patent, any subject, including hemodialysis patients, has a set level of sodium in his body, referred to as the “set point.” The set point of a subject tends to remain relatively constant, and sodium levels deviating too far from the set point may cause discomfort to the subject. Given the above, the method of the prior art includes causing blood to flow through blood conduit of the dialyzer and flowing the conductivity-testing solution in the opposite direction through the dialyzer.
Conductivity detectors measure the conductivity of conductivity-testing solution as the solution enters and exits dialyzer. Conductivity-testing solution is formulated such that electrically conductive solutes other than sodium in the patient's blood have little or no effect on the conductivity measurements of conductivity-testing solution.
According to U.S. Pat. No. 8,182,692, due to the closely matched concentrations of electrically conductive solutes, such as phosphate, sulfate, bicarbonate, potassium, calcium, and magnesium, in conductivity-testing solution and in the patient's blood, little diffusion of those electrically conductive solutes occurs across membrane. Consequently, the conductivity measurements of conductivity-testing solution are closely correlated with the level of sodium in the patient's blood.
Therefore, conductivity-testing solution is exclusively used to accurately determine the level of sodium in the patient's blood as a function of the change in conductivity across dialyzer of the conductivity-testing solution.
Control unit then determines the level of sodium in the patient's blood as a function of the measured conductivity values.
After determining the concentration of sodium in the patient's blood, dialysis fluid may be prepared to include a concentration of sodium that is substantially equal to the concentration of sodium determined to exist in the patient's blood.
Moreover, US2012/018379 discloses an apparatus and a method for the determination and regulation of the concentration of one dissolved substance (e.g. sodium) in a dialysis fluid circuit of a hemodialysis machine.
The user presets the sodium regulation range before the start of the dialysis using an estimated value for the dialysis fluid sodium required to achieve the isonatremic state or a lab measurement of the patient sodium or a value determined by the regulation from earlier treatments. In addition, the distribution volume of the patient is input for the application of the model for the correction of the diffusive balance. Furthermore, the initial concentrations of bicarbonate and potassium in the patient are set. They come from an analysis by means of a blood gas analyzer before the start of the dialysis treatment.
After the start of the treatment, the dialysis fluid flow and the conductivity are determined upstream and downstream of the dialyzer and a calculation of the updated current bicarbonate and potassium concentration in the patient takes place with it being assumed that the potassium clearance corresponds to the sodium clearance and that the bicarbonate clearance corresponds to 70% of the sodium clearance. The sodium clearance from the blood flow is estimated until the presence of the first clearance measurement.
The calculation of the conductivity balance and of the correction term for the ion exchange and thus for the sodium balance then takes place from these data.
The conductivity of fluids measured upstream and downstream, the sodium balance and the correction term for the dialysate conductivity downstream of the dialyzer are then the input values for the sodium regulation. The desired conductivity thus determined is finally converted into a desired value for the dialysis fluid sodium while taking account of the composition of the dialysis concentrate and this preset value is transmitted to a metering unit for dialysis fluid preparation.